Methods for the continuous preparation of pourable, particularly thermoplastically shapable, tempered material, in particular PVC powder mixture, for the so-called hot charging of processing machines are known. It is the purpose and aim of these known methods to use the thermal energy introduced into the material during the preparation to a dry blend or agglomerate as loss-free as possible by the immediately following further processing.
In the methods of this type being most customary today the heating of the material to be mixed to the necessary final preparation temperature during the mixing procedure is effected almost exclusively by means of the mechanical energy introduced into the material to be mixed by means of a rapidly running mixing tool in the so-called hot mixer or turbo mixer. However, methods are also known, in which the hot mixer has slowly running mixing tools, by means of which approx. only one third of the heat quantity enters the material to be mixed via mechanical energy, the remainder is introduced into the material to be mixed as thermal energy via the heated mixer wall. In these known methods a so-called heat retention mixer with additional heat exchange surfaces in the form of heatable coils, rings or ring segments is in rear position to this mixer. In this tempering or heat retention mixer a very slowly running mixing tool is rotated, which moves the material to be mixed past the inner surfaces of the container or the installations, rearranges the same constantly and thus keeps it at admission temperature. The outlet of this heat retention mixer is provided with a worm conveyor, which conveys the heated material into the processing machine.
Another known method uses three rapid mixers having approximately the same design, which have one rapidly running mixing tool each, one inlet each in the middle of the cover and one outlet each at the edge of the container and which are assembled in a stepped manner (German Pat. No. 1 778 436). The material continuously metered into the first mixer is also here heated by frictional heat. The warmer particles of the material are to rise towards above in the fluidized material and fall into the centre of the next mixer through the outlet. This procedure is repeated in the other mixers. A metering screw is connected to the outlet of the last mixer, which conveys the material thus heated in stages into the processing machine.
The disadvantages of this known process are that the introduction of heat into the bulk material by means of friction is relatively non-economical and that some raw materials do not withstand such violent treatment over a longer period of time without damage. A very essential disadvantage is furthermore that an especially uniform temperature distribution cannot be achieved within the material. However, said temperature distribution is in most cases indispensable in order to achieve a high quality in the product produced from this raw material. Thus it is e.g. necessary to keep the processing temperature .+-.1.degree. constant in the PVC during extrusion in the range of 458.degree. K. in order to achieve maximum notched bar impact resistance. No such uniform temperature homogeneity can be achieved within the material with the known preparation methods that the above-mentioned temperature range can always be observed during extrusion. This is the reason why the above-described preparation methods did not prevail in practice. A homogenizing which can possibly be achieved by a long-lasting circulation of the material is out of the question in practice, since PVC can only be kept stable economically at high temperatures during a relatively short period of time.
Therefore one started to cool the PVC prepared to a dry blend again to room temperature and to supply the material without preheating to the further processing. However, then an extruder must e.g. be designed in such a manner that it alone undertakes the reheating of the material to the temperature necessary for extrusion. Such an extruder is relatively long and must have a relatively heavy construction. Analogous reflections apply to calendering and other processing machines. The described cooling must take place rapidly due to the limited period of time which PVC remains stable in hot state and thus requires the use of special devices.
Recently it has been discussed again to supply a re-preheated raw material to the extruder during extrusion. It is obvious that here one is again confronted with the problem of temperature homogeneity, the maximum treatment time must be observed and that the process is rather energy-wasting.